Woven laminates of the initially mentioned type are known from the state of the art. So called microporous cover materials are used for the sound absorption in hot gas flows. This is in particular always the case when the frequencies to be absorbed are comprised in the range above 800 Hz up to 6000 Hz, as they occur in turbines or other fast running rotary systems, such as for example in auxiliary gas turbines (APUs) of aeroplanes or the like.
The functioning of the microporous materials is to convert the energy of sound pressure waves into thermal energy. For this, it is necessary to cause a certain pressure loss by means of the microporous material. The partially high temperatures of the combustion gases of gas turbines of up to 900° C. require a corresponding material on metallic or ceramic base. The large field of metallic materials offers different possibilities for this.
One of the most famous solutions is the use of sintered metal fibre fleeces, such as they are also used for the purpose of the solid/liquid filtration. Such metal fibre fleeces are inserted in cylindrical form as a lining into a sound absorber section of an APU. Herein, the high weight of these fleeces as well as also their further processing by welding, which is extremely complicate, presents a substantial disadvantage. Another disadvantage are the long sintering times which lead to high costs and the brittle behaviour of the sintered material. This is essentially caused by the material to be selected, which always comprises a strong oxide forming alloying addition due to the high temperatures. A typical material for this is an alloy of FeCrAlY. A metal fibre fleece having sound absorbing properties is known from DE 102004018810 A1.
Therefore, new developments intend to substitute these metal fibre fleeces by woven textiles or woven laminates. These ones are single or multi-layer woven wires that are combined to solid woven laminates by means of a sintering process. Typically, one of the woven layers acts as acoustically active layer because of the selected microporous mesh.
Furthermore, reinforced metal fibre mats are known from the state of the art, such as for example from DE 12 98 507 A. The here described metal fibre mat consists of overlapping metal fibres that are arbitrarily distributed in a mated body and connected to each other by in particular sintering, wherein the particularity is that a permeable metal insert, that is provided with a greater free screen section than the mat, is embedded in and metallurgically connected to at least one surface of the metal fibre mat. The thus formed metal fibre mat serves in particular as insert for liquid filters.
Furthermore, a combined band for sound absorption, filter and padding purposes is known from the German Utility Model 19 68 840. This band is composed of two or more types of appropriate fleece-like material or wire mesh, wherein the inner core region is formed by glass, mineral or rock wool and the outer zone is formed by steel wool and/or wire mesh.